If you create another project and it exports out fine with different media, that would be one indication that there is something amiss with your non-exporting project. Try a new project with the same media as well. That would tell you whether it is the media that is the problem.
Also, try slowly scrolling through your timeline in the troublesome project. Look for black frames, white flashes, artifacts -- anything that might indicate corruption.
Are the clips in an odd or outdated video or audio format, such as .avi?
If you can export one of the clips to your Mac desktop you can try using the free download, Handbrake, to convert it to Mp4/AAC. Do that even if the clip already is Mp4/AAC. Sometimes reconverting can shake out issues.
You can get Handbrake here:
https://handbrake.fr/
A simple way to do it is to open Handbrake and do a File/Open Source. Navigate to your video and choose it as the source from the resulting screen. Then do File/Start Encoding. Wait a couple of minutes for the conversion to
complete. Then save and import the converted clip into iMovie.
-- Rich